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Low-Mg and Fe-rich tholeiitic dykes of basaltic andesite to andesites are common in many Precambrian cratons and they develop under arc-related tectonics and also due to the melting of the pyrolitic mantle. Such equivalent rock types are well embedded in the older trondhjemite gneisses of Sukma–Bengpal group in the southern part of Bastar Craton, Central India. These are well demarcated around the Mesoproterozoic Indravati and Sukma basins of the craton, occurring in the form of thin dykes and veins, trending parallel as well as diagonally to the NW–SE and ENE–WSW striking brittle–ductile shear zones. Their petrography shows that they are dominant with ferro-hornblende and magnesio-hornblendes, along with plagioclase, chlorite and accessory minerals of magnetite and ilmenites resembling the older amphibolite variety. The mineral chemistry of amphiboles shows that they are sub-alkaline in nature and crystallised under high oxygen fugacity conditions. The hornblende-plagioclase thermo-barometry indicates that these were re-equilibrated at temperatures of 600–650°C within the low pressures of 3–5 kbar under greenschist to lower amphibolite facies of metamorphism. The whole-rock chemistry represents the andesitic nature of sub-alkaline Fe-rich tholeiitic type. The primitive mantle and MORB normalised spider diagrams show strong enrichment of LIL elements (K, Rb, Ba, Th) and depletion of HFSE (Zr, Hf, Ti, Yb, Sc) with negative Nb anomalies. The above geochemical data on various tectonic discrimination plots reveal that they originated under continental arc volcanism of subduction-related tectonics and were derived from the spinel-garnet lherzolite melting source as similar to various metabasalts and arc tholeiites described from several parts of the craton. [ABSTRACT FROM AUTHOR]

Alaskan-type of plutonic complexes represent arc magma chambers and they occur throughout the earth's history from Archean to Phanerozoic periods around the globe. The ultramafic rocks from these complexes commonly contain spinels of varied chemistry, especially rich in Cr, Fe, and Ti coupled with low-Al contents when compared to the spinels from oceanic or continental rift domains. An attempt is made here to study Thenmudiyanur Mafic–Ultramafic Complex (TMUC) from the Belur Shear Zone (BLSZ) in the northeastern margin of Southern Granulite Terrane, located about 20 km SW of Tiruvannamalai Town. The important lithologies of TMUC include dunites, peridotites and pyroxenites surrounded by metagabbros and amphibolites. The dunites and peridotites are highly serpentinized with well-preserved cumulate olivines along with magnesite, chrome spinel and clinopyroxenes. The mineral chemistry of olivines represents higher forsterite (Fo88–90) and Mg# 87–90 [100×Mg/(Mg+Fe2+)] contents. The gradational contact within the rock types, compositional variations and cumulate nature of mineralogical characteristics suggest fractional crystallization of magma from a single parental source. The chrome spinels from these lithologies represent higher Cr# [100×Cr/(Cr+Al)] that ranges from 65 to 86 along with lower MgO (1.06–4.57 wt%) and MnO (<1 wt%) contents. The presence of chrome spinels together with olivine in dunites as well as peridotites and clinopyroxene mineral chemistry from pyroxenites reveal arc signatures for the evolution of TMUC, which seems to be similar to that of the Alaskan-type. Considering the tectonic setting and the age relationships of adjacent lithologies, it is opined that the TMUC might have evolved during the Neoproterozoic period. [ABSTRACT FROM AUTHOR]

The Chalk hills of Salem complex also called Salem mafic and ultramafic complex (SMUC), occurs in northern part of Cauvery suture zone (CSZ), southern granulite terrane (SGT), India with the lithological sequence of ultramafic cumulates, mafic intrusions of gabbros and amphibolites with quartzo-feldspathic veins and ultrapotassic dykes. Recently the complex has been described as Neoproterozoic Alaskan-type. The Ti- rich gabbros are in association with serpentinized dunites and pyroxenites in the form of several intrusions at various locations of the complex. The petrography of these gabbros shows sub-alkaline to alkaline in nature with mineralogy of two pyroxenes and plagioclases of calcicsodic assemblages along with magnetite-titanomagnetite-ilmenite (Ti-spinels). The whole rock geochemistry indicates that they are tholeiitic in nature with enrichment of LILE (Rb, Ba, Th, Sr) and depletion of HFSE (Nb, Ta, Ti) with reference to N-MORB. The mineral chemistry of clinopyroxenes from these gabbros show slightly higher Mg# (0.68–0.80), TiO2 (0.40–0.73 wt%) with lower Al2O3 (4.4–5.5 wt%), Na2O (0.53–0.94 wt%) and Cr2O3 (0.1–0.17 wt%) contents. The orthopyroxenes also show low Al2O3 (1.4–2.3 wt%), Cr2O3 (up to 0.13 wt%), and relatively higher MgO (17.11–21.79 wt%). The Ti- bearing oxides (ilmenites) of these gabbros are characterized by higher contents of TiO2 (38–51 wt%) and FeO (46–56 wt%), similar to that of Ti-Fe rich Alaskan- type of mafic rocks. On various tectonic discrimination diagrams of whole rock and clinipyroxene mineral chemistry of these gabbros reveal that they are derived from subduction related components of arc magmas of titanium rich, evolved through the process of high degree of crystal fractionation. From the two-pyroxene thermometry, the temperature of formation of these gabbros are determined as 893–1014°C and the clinopyroxene barometry results variable pressures of 8–13 kbars. The tectonic setting with the available age relationships from the complex suggest these intrusions are syntectonic and co-genetic of late emplacements along with the ultramafics during Neoproterozoic subduction events. Such occurrences of high Ti- bearing gabbros are common in many Alaskan- type of complexes like Neoproterozoic Arabian Nubian Shield and other parts of the world. [ABSTRACT FROM AUTHOR]

The Mahadevi hills, located in the axial zone of Cauvery Suture Zone, comprise a sequence of granulite facies rocks represented by garnet-bearing pyroxene granulites and quartzo-feldspathic gneisess interfolded with banded iron formations. Structural mapping with hand held GPS reveals that the Mahadevi hills constitute a mega sheath fold structure exposing well developed easterly plunging extension lineations. Depressional and culmination surfaces are well demarcated in association with elliptical map patterns. The development of the mega sheath fold structure is genetically related to the regional thrust-nappe tectonics, supporting the model of subduction-accretion-collisional history for the evolution of the Cauvery Suture Zone. [ABSTRACT FROM AUTHOR]

A new map of structural architecture has been compiled involving modern mapping techniques at the cratonmobile belt interface in the Western Ghats around the Coorg granulite massif revealing the occurrence of important shear zones. The shear zones are linked to the Moyar-Bhavani Shear Zone in Southern India. The nature, geometry and kinematics of the shear zones in the granulitic crust and the cratonic part are distinctly different. [ABSTRACT FROM AUTHOR]

Occurrence of two lamproitic dykes intruding the basement granite near Khadka village at the northern margin of the Indravati Basin, Bastar craton is reported. Combined field, microscopy, XRD, EPMA and whole-rock geochemical investigations reveal that these lamproites were subjected to high degree of hydrothermal alteration as well as possible metamorphism. However, relicts of their original textures are well-preserved thereby providing important clues as to the nature of the protolith. Quartz, carbonate, chlorite and phlogopite constitute the bulk mineralogy whereas spinel, apatite and iron oxides are the accessory phases. Chemical composition of the groundmass spinels are strikingly similar to those from the lamproites. The Khadka lamproite dykes display high abundances of compatible elements such as Ni (238-396 ppm), Cr (484-892 ppm), and V (160-200 ppm) as well as high-field strength elements such as Zr (719-2057 ppm) and Nb (92-126 ppm) that resemble those in lamproites. Khadka lamproites also have high whole-rock REE abundances (ΣREE up to 1260 ppm) and display fractionated chondrite-normalized REE patterns (La/Yb= 113-237) which together with their average compatible and incompatible trace elemental ratios (e.g. Nb/Zr, Nb/La, Ba/Rb) are strikingly similar to those of the Mesoproterozoic Krishna lamproites of the Eastern Dharwar craton. Available field evidences suggest the Khadka lamproites to be of at least Palaeoproterzoic age (1.88 Ga) which makes them some of the oldest such rocks as yet documented from the Indian shield. [ABSTRACT FROM AUTHOR]